In general, a memory device refers to a device capable of storing and retaining predetermined data and reading the data at a required time point. An example of the memory device may be a dynamic random access memory (DRAM) device. A plurality of unit cells, each unit cell including one transistor and one capacitor, are integrated in the DRAM. That is, I-bit data is stored depending on whether or not charges are charged in the capacitor of the unit cell.
In recent years, research has been conducted on a technique of storing a larger amount of data by integrating many unit cells on the same area to ensure price competitiveness. In order to integrate a larger number of unit cells on the same area, the integration areas (i.e., sizes) of a transistor and a capacitor constituting a unit cell should be reduced. However, the areas of the transistor and the capacitor cannot be infinitely reduced.
A reduction in the area of the capacitor leads to a reduction in capacitance. Thus, the height of the capacitor should be increased to maintain the constant area of the capacitor. That is, for example, when the design rule of a DRAM is 60 nm, the capacitor has a height of about 1.6 μm. If the design rule of the DRAM is reduced to 40 nm, the height of the capacitor is increased to about 2.0 μm. When the height of the capacitor is increased, since an aspect ratio is increased during formation of holes required to manufacture a cylindrical capacitor, efficient patterning may be precluded. Also, since an interval between adjacent capacitors is reduced and the height of the capacitors is increased, the capacitors collapse and the adjacent capacitors may be electrically connected to one another. Accordingly, when the design rule of the DRAM is reduced to 40 nm or less, applying the cylindrical capacitor is difficult.